I’m not a hardware engineer, but my understanding is that a USB device isn’t supposed to draw that much current without performing either negotiation or sensing. Including a note that the user must use a 1A charger suggests that the device is violating the USB spec and will try to pull 1A no matter what. It then makes me worry about what other power safety systems are absent, since it’s a device with a lithium battery that gets strapped directly to a body part.
What is the specific unsafe condition are you worried about? I don’t know everything but I did evaluate consumer tech for Underwriters Laboratories to IEC 60950 and 62358. I think your caution is commendable but nothing you’ve described is inherently wrong. USB is just a connector. When the product said it needs 5w (I assume you mean 1A @ 4.8-5.1Vdc) by itself means nothing without more information.
One really obvious point, did the instructions say it’s a) dangerous with a power supply that can’t provide 1A or b) not/less functional without the 1a supply? If they were not specific, I’m happy to easy your mind - it was the latter.
> did the instructions say it’s a) dangerous with a power supply that can’t provide 1A or b) not/less functional without the 1a supply?
To me, the real red flag is: what happens when you plug it into an 3 A USB charger? Being a constant voltage supply, the load should only draw as much current as needed, and it should work with no problems (which is the usual expectation of 90% of consumer electronics with a USB port). The warning of "only use 1 A charger" notice is suspicious because it may suggest that the product doesn't include any internal overcurrent protection by itself (like a $0.1 polyfuse), and solely depends on the current limiting of the USB charger for its safety. While it's possible that it's not inherently wrong, but it's certainly a bad design practice.
The specific unsafe condition I am worried about is that the power management electronics lack any sort of fault protection, and this ends up causing a shock, burn, or fire.
I have a hard time imagining a situation where a wearable with a USB charging port is designed so cheaply that it only works with specific USB chargers, but still includes over-temp, over-volt, over-current, etc. protection.
Needless to say, there is not even a suggestion of this thing being either UL or USB-IF certified.
